[clang] [clang][SME] Ignore flatten for callees with mismatched streaming attributes (PR #116391)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic approved this pull request.

Oh, right, we have isInlineViable(), but attributes aren't part of it for... reasons.  (I haven't tried to dig into https://discourse.llvm.org/t/rfc-avoid-inlining-alwaysinline-functions-when-they-cannot-be-inlined ; I'll believe you that it's non-trivial to fix.)  Hence, shoving this into the frontend.

I guess this is fine, then.  But please add a note describing the current situation, so the next person looking at this has context.  (Put it in TargetInfo.h, maybe?) 

LGTM with the note.

https://github.com/llvm/llvm-project/pull/116391